The regulation of phosphodiesterase (PDE) in rat carotid body was studied in response to chemosensory stimulation. Sufficient exposure to hypoxia causes an increase of Ca2 ion binding protein in the cytosol fraction of carotid body homogenates. PDE occurs in two kinetic forms--one with a high K and the other with a low Km for cAMP. After exposure to hypoxia only a low Km form of PDE could be detected, indicating an activation of the enzyme. This activation of PDE was prevented by carotid sinus nerve transection prior to hypoxia or by the addition of EGTA to the incubation media. BIBLIOGRAPHIC REFERENCE: Hanbauer, I. and Lovenberg, W.: Presence of a calcium 2 ion dependent activator of cyclic-nucleotide phosphodiesterase in rat carotid body: Effects of hypoxia. Neuroscience, 1977 (In press).